The results rule out a substantial portion of parameter space for new forces with a range between one-tenth and one-hundredth of a millimeter, where theoretical physicists using string theory have proposed that "moduli forces" might be detected, according to the researchers.

In string theory, which is considered the most promising approach to the long-sought unified description of all known forces and matter, everything in the universe is proposed to be composed of tiny loops of vibrating strings.

"Our results represent the most sensitive search for new forces at this length," said lead author Joshua Long, a former postdoctoral researcher in the lab of CU-Boulder physics Professor John Price. Long now works at the Los Alamos Neutron Science Center in Los Alamos, N.M.

A paper on the subject by Long, Price, Allison Churnside, Eric Gulbis and Michael Varney of CU-Boulder will appear in the Feb. 27 issue of the journal Nature.

In order for string theory to work, there must be six extra spatial dimensions beyond the three that are observable, and theorists believe these extra dimensions are curled up into small spaces. This "compactification" creates what are called moduli fields, which describe the size and shape of the compact dimensions at each point in space-time, according to Price.

Moduli fields generate forces with strengths comparable to gravity, and according to recent predictions might be detected on length scales of about one-tenth of a millimeter.

"If these forces exist, we now know they have to be at even smaller distances than we have measured here," said Price. "However, these results don't mean that the theories are wrong. Researchers will just have to measure at even shorter distances and with higher sensitivity."

The experiment uses two thin tungsten reeds. One of them is moved back and forth so that the gap between the two reeds varies at a frequency of 1,000 cycles per second, according to Price.

Motions caused by forces on the second reed are detected with highly sensitive electronics. The experiment can detect forces as small as a femto-newton, or about one-billionth of the weight of a grain of sand, he said.

Price said he will continue conducting experiments to try to measure even shorter distances next.

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